This invention relates to processes for manufacturing polycrystalline bodies. These bodies may be comprised of polycrystalline diamond, polycrystalline cubic boron nitride, and polycrystalline wurzitic boron nitride, as well as combinations thereof. For convenience in the following discussion and claims the terms CBN and cubic boron nitride are intended to refer to both of the cubic and wurzitic high pressure forms of boron nitride. In addition, as used in the following specification and claims, the term "polycrystalline" refers to the type of structure which is characterized by substantial intercrystalline bonding between adjacent crystals.
In recent years the manufacture and use of polycrystalline diamond (PCD) and polycrystalline CBN (PCBN) in various applications has become well established. In most of these applications the PCD or PCBN body is used as a distinct cutting element or wear part and is held and supported by some type of tool.
A contrasting use of diamond is to incorporate single crystal diamonds in a metal matrix to make matrix cutters for use in concrete saws, core drills and the like. The sizes used for these matrix cutters are typically between 100 and 10 U.S. mesh.
The use of single crystal diamonds in a metal matrix presents certain disadvantages. For one, the sizes of diamond crystals suitable for metal matrix use are expensive to obtain. This is so because the cost of natural and synthetic diamond increases with size and quality. A second disadvantage results from the fact that diamond crystals can be fractured with relatively low impact force when that force is applied parallel to the crystals' planes of cleavage. Naturally, such fracturing causes premature loss of the crystal to use. A third disadvantage is the poor attachability of the diamond crystals to the matrix. In particular, because single crystal diamond has a relatively smooth surface that is virtually non-wettable, mechanical forces must be relied on to hold a crystal in the matrix. As a result, when too much of the matrix material wears away and exposes more than half of a particular crystal, that crystal will become disattached and, of course, lost to use.
It has occurred to the inventors that using polycrystalline diamond or polycrystalline CBN in the place of single crystal diamond could ameliorate these problems. First, the cost of obtaining these sizes of PCD pieces should be less than that of obtaining single crystals. Second, the impact resistance of PCD is greater than that of single crystal diamond because of the random orientation of the many individual crystals in PCD. Third, it ought to be possible to produce PCD in predetermined shapes which would have enhanced attachability in the matrix.
Unfortunately however, a satisfactory method of obtaining PCD or PCBN bodies which could be used in this application, to the best of the inventors' knowl edge, was not available in the prior art. That is, a method was not found in the art for efficiently producing large quantities of well-shaped 100 to 10 U.S. mesh PCD or PCBN pieces.
One possible method is to produce larger pieces of PCD or PCBN and then to cut them to the desired size. However, this is inefficient because PCD is relatively hard to cut. Although PCD may be cut with an electric discharge machine, the process is expensive in terms of capital, energy, and labor. For example, the inventors have observed that 10 to 20 carats of 25 mesh PCD requires about 24 hours to cut.
Another method which has been tried is to crush larger bodies of PCD or PCBN into smaller pieces. One problem with this method is that a range of shapes and sizes are produced in the crushing process. That is, pieces both above and below the desired size are produced. Also, the shapes are irregular and tend to be elongate rather than blocky as desired for matrix applications.
Another method is to mold PCD or PCBN pieces in a process such as that described in U.S. Pat. No. 3,819,814 wherein quantities of crystals with a catalyst/binder are placed in a rigid mold made from a material such as cemented tungsten carbide or graphite. However, because the pieces required are so small and the quantity desired from each press run is so high, this type of molding is likewise impractical. In addition, the molds, which can only be used once, are relatively expensive due to their intricateness together with their chemical and physical requirements.
Another molding technique is taught in the European Patent Application No. 071,036. In this method, the reaction chamber is divided by partitioning strips of metal. In this way, multiple pieces of PCD or PCBN are produced. However, the number and minimum size is limited in this method. That is, it would be difficult if not impossible to provide the large number of extremely thin partitions needed to efficiently produce large quantities of these smaller sized PCD or PCBN pieces.